In a multi-level QAM system, information is included in both phase and amplitude. In the multi-level QAM system, it has been thought customary to transmit a predetermined signal for phase and amplitude synchronization. In HSPDA in 3GPP (Third Generation Partnership Project), a synchronization signal along the phase direction is transmitted over a separate channel. However, no synchronization signal along the amplitude direction is transmitted. Moreover, the transmission power is varied with time in accordance with the operation in a base station. Thus, there arose a necessity for the mobile station to estimate a threshold value to achieve amplitude synchronization, without resorting to a preset signal. The outline of the HSPDA are now explained (see for example the following Patent Publication 1).
The HSDPA, in which a base station transmits high-speed data via a downlink network to a mobile station in a cellular system is now under investigation by 3GPP. In this HSDPA, a high speed physical downlink shared channel (HS-PDSCH) is used for transmission over downlink channel from the base station to the mobile station. This HS-PDSCH is used for data transmission from each base station to a plural number of mobile stations. Thus, the base station or a base station control device decides on a schedule for transmitting data to each of the plural mobile stations and transmits data at timings which differ from one mobile station to another. For controlling the data transmission from the base station to the mobile station, each base station sets a dedicated channel DPCH (dedicated physical channel) independently with each of the mobile stations. This DPCH is used for transmitting control information from the base station to the mobile station by a downlink signal and for transmitting control information from the mobile station to the base station by the signal in the reverse direction, that is, by an uplink signal. The control information transmitted from the base station to the mobile station includes the information on the data transmission timing to the mobile station.
In the HS-PDSCH, there is known a technique in which such a modulation system is selected and used, depending on the state of the propagation channel between the base station and the mobile station, from among plural modulation systems, such as QPSK, 16-ary QAM or 64-ary QAM, which will enable the fastest data transmission, insofar as the target bit error rate is satisfied. The information for selecting the modulation system is also transmitted from the base station to the mobile station as the control information. There are occasions wherein, for changing over the modulation mode, the receiving quality of common pilot signals, transmitted from the base station, is measured, and the measured results are transmitted as the control information from the base station to the mobile station. In the mobile station, the ratio of time spent by the mobile station on receiving data with use of the HS-PDSCH is small. However, the DPCH is persistently allocated between the mobile station and the base station, even in the data awaiting state in which data is not received, so that data transmission can be commenced in a short time when a request is made for data transmission. Thus, although each base station may have data communication with only one mobile station at the same time, a large number of mobile stations are in the data awaiting state, and set DPCH between the base station and the mobile stations.
In the data transmission which employs HS-PDSCH, if the control information transferred by DPCH is low in reliability, the occurrence of reception error in the control information received by the base station and the mobile station is increased, thereby to lower the data transmission efficiency. In the HS-PDSCH, transmission power is set larger than that of a downlink signal of each DPCH, in order to effect high-speed data transmission. Thus, if the data block is re-transmitted due to failure in transmission, the power of an interference wave of the downlink is increased appreciably, thus reducing network capacity.
As for details in the signal format of the HS-PDSCH (sub-frame structure), reference is made to publications listed below (for example, see non-patent publications 1 and 4). As for the constellation chart for 16-level QAM and the spreading of the downlink physical channel, reference is made to publications listed below (for example, see non-patent publications 2 and 5). As for the power control of HS-PDSCH, reference is made to publications listed below (for example, see non-patent publications 3 and 6). As for the structure of phase synchronization for 16-level QAM on the CPICH, reference is made to publications listed below (for example, see non-patent publication 1). The CPICH is transmitted by a specified code (for example, see non-patent publication 2).
Non-Patent Publication 1:
3GPP TS 25.211 V5.1.0. (2002 June) (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD) (Release 5)) 5.3.3.13 High Speed Physical Downlink Shared Channel (HS-PDSCH), 5.3.3.1, Common Pilot Channel (CPICH) Internet URL <http://www.3gpp.org/ftp/Specs/2002-06/Rel-5/25-series/ File name: 25211-510.zip
Non-Patent Publication 2:
3GPP TS 25.213 V5.1.0. (2002 June) (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Spreading and modulation (FDD) (Release 5)) 5.1 Spreading, 5.2.2 Scrambling Code Internet URL <http://www.3gpp.org/ftp/Specs/2002-06/Rel-5/25-series/ File Name: 25213-510.zip>
Non-Patent Publication 3:
3GPP TS 25.214 V5.1.0. (2002 June) (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 5)) 5.2.11 HS-PDSCH Internet URL <http://www.3gpp.org/ftp/Specs/2002-06/Rel-5/25-series/ File Name: 25214-510.zip>
Non-Patent Publication 4:
3GPP TS 25.211 V5.2.0. (2002 September) (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD) (Release 5)) 5.3.3.13 High Speed Physical Downlink Shared Channel (HS-PDSCH), Internet URL <http://www.3gpp.org/ftp/Specs/latest/Rel-5/25-series/ File Name: 25211-520.zip>
Non-Patent Publication 5:
3GPP TS 25.213 V5.2.0. (2002 September) (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Spreading and modulation (FDD) (Release 5)) 5.1 Spreading, 5.2.2 Scrambling Code Internet URL <http://www.3gpp.org/ftp/Specs/latest/Rel-5/25-series/ File Name:25213-520.zip>
Non-Patent Publication 6:
3GPP TS 25.214 V5.2.0. (2002 September) (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 5)) 5.2.11 HS-PDSCHS Internet URL <http://www.3gpp.org/ftp/Specs/2002-06/Rel-5/25-series/ File Name: 25214-520.zip>
Meanwhile, there is known an n-ary (multi-level) QAM decoding apparatus, as a decoding apparatus for preventing an error in data decision in decoding a wireless communication signal of the orthogonal frequency division multiplexing (OFDM) system employing a demodulation system for a multi-level QAM signal, in which frequency domain signals of the baseband Ich and Qch are Fourier-transformed, variations in the amplitude and the phase of Ich and Qch data signals are estimated, in the estimating unit for Ich and in the estimating unit for Qch, respectively, based on a pilot signal in the transformed signals, and in which the threshold values are corrected in an Ich threshold correction unit and a Qch threshold correction unit, based on the estimated results (for example, see Patent Publication 2).
Moreover, in a fading network where the transmission line is subjected to severe variations, there is known a configuration including a transmission line distortion compensation unit, which is adapted for estimating the threshold value information as needed for deciding data in a decoding unit, as a transmission line compensation system in case of managing an n-ary(multi-level) quadrature amplitude modulation system (for example, see Patent Publication 3).
Patent Publication 1
Japanese Patent Kokai Publication No. JP-P2002-325063A (page 5)
Patent Publication 2
Japanese Patent Kokai Publication No. JP-P2002-217862A (pages 3 and 4, FIG. 2)
Patent Publication 3
Japanese Patent Kokoku Publication No. JP-B-6-1908 (pages 2 to 4, FIG. 4)